[iOS][Signin] Fixing font issues on the MDCButton

With a MDCButton, the font should not be set on the title label, but on the
button itself, using:
-[MDCButton setTitleFont:forState:]

Bug: 803767
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I4232f2d080daca4e8fc2e585d51c24c1b10ab1ac
Reviewed-on: https://chromium-review.googlesource.com/891221Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533067}

ios/chrome/browser/ui/authentication/chrome_signin_view_controller.mm

@@ -840,10 +840,10 @@ enum AuthenticationState {
 }
 
 - (void)layoutButtons:(const AuthenticationViewConstants&)constants {
-  [_primaryButton titleLabel].font =
-      [[MDCTypography fontLoader] mediumFontOfSize:constants.SecondaryFontSize];
-  [_secondaryButton titleLabel].font =
+  UIFont* font =
       [[MDCTypography fontLoader] mediumFontOfSize:constants.SecondaryFontSize];
+  [_primaryButton setTitleFont:font forState:UIControlStateNormal];
+  [_secondaryButton setTitleFont:font forState:UIControlStateNormal];
 
   LayoutRect primaryButtonLayout = LayoutRectZero;
   primaryButtonLayout.boundingWidth = CGRectGetWidth(self.view.bounds);

ios/chrome/browser/ui/first_run/welcome_to_chrome_view.mm

@@ -555,8 +555,9 @@ const char kPrivacyNoticeUrl[] = "internal://privacy-notice";
 }
 
 - (void)configureOKButton {
-  self.OKButton.titleLabel.font = [[MDCTypography fontLoader]
+  UIFont* font = [[MDCTypography fontLoader]
       mediumFontOfSize:kOKButtonTitleLabelFontSize[self.cr_widthSizeClass]];
+  [self.OKButton setTitleFont:font forState:UIControlStateNormal];
   CGSize size = [self.OKButton
       sizeThatFits:CGSizeMake(CGFLOAT_MAX,
                               kOKButtonHeight[self.cr_widthSizeClass])];